1. Field of the Invention
The present invention is directed to international wireless communication and, more particularly, to a method and apparatus for facilitating service to international roamers in a cellular system.
2. Background
Providing immediate access to home and work from remote locations, cellular telephones ("cell phones") have become increasingly popular. No longer prohibitively expensive, cell phones are used by a wide range of consumers. Presently, there are over 75 million cellular customers worldwide. Predictions show that over 300 million customers will be using cellular systems by the year 2,000. Cell phones and systems that are being introduced to the market have advanced features, services, and cost advantages over the older cellular technologies.
A cellular customer (a "subscriber") obtains cellular service within a region by subscribing to a wireless service provider ("WSP") that is licensed to provide service for that region. A single WSP uses many small radio coverage areas ("cells") to serve hundreds of square miles. The WSP relies upon a Mobile Switching Center ("MSC") to interconnect small radio coverage areas into a larger system as well as accepting and delivering calls to other networks, namely the public switched telephone network (PSTN). To maintain a call when the cell phone moves to another cell, the WSP switches the phone's radio channel frequency to another frequency available for use in an adjacent cell site. A frequency may be reused by more than one customer at a time in different cells without subscriber interference if they are far enough apart. WSPs take advantage of this by breaking the coverage area into many small cells. Adjacent cells use different frequencies to avoid interference, but widely separated cells may reuse the same frequencies. This allows the WSP to repeatedly reuse radio channels and increase the number of subscribers they can serve with a limited number of channel frequencies.
Each cell phone (or "mobile station") uses a unique signaling identifier when operating in a cellular system. This information is stored in its internal memory, called the Number Assignment Module ("NAM"). The NAM also contains a Mobile Identification Number ("MIN"), a home system identifier, access classification, and other information. The MIN value is usually the same value as the subscriber's mobile directory number ("MDN"), also known as the telephone number. The format of the telephone number for the United States, Canada, and various U.S. territories is based on the North American Numbering Plan ("NANP"). This plan defines a format based on a three digit Numbering Plan Area ("NPA"), a three digit central office code ("NXX"), and a four digit subscriber line number ("XXXXs"). (This is frequently denoted using a NPA-NXX-XXXX format where N=digits 2 through 9 and X=digits 0 through 9.) Since the MIN is the same value as the MDN, it is also implemented as a ten digit number that follows the NPA-NXX-XXXX format.
WSPs often allow customers from other cellular systems to use their service. The ability of a subscriber to use the service of a different WSP is termed "roaming". A common type of roaming is "automatic" roaming and is initiated by a process termed registration. Roamer registration occurs as follows: When a mobile station detects a foreign cellular system, it attempts to register with the system. This is accomplished by signaling, among other things, its MIN value and electronic serial number. The serving system searches for the wireless telephone's MIN in its visitor location register ("VLR") and determines that the wireless telephone is not yet registered. The visited cellular system uses the wireless telephone's MIN and ESN to request validation of the subscriber's identity from the subscriber's home cellular system, specifically the Home location register ("HLR"). The HLR will then determine if the subscriber is valid and acknowledge the status to the requesting cellular system. The visited VLR then temporarily stores the subscriber's registration information for future authorization eliminating the need to contact the subscriber's home HLR again if a call is processed. Once validated, the mobile station can receive and originate calls transparently. After a predetermined period of inactivity, the temporary authentication information stored in the VLR is erased.
The communication between the VLR and HLR frequently uses a signaling network known as Signaling System 7 ("SS7"). Each of the entities on this network, for example the VLR and HLR, is identified by a unique address called a point code. In many instances, a separate network entity called a signal transfer point ("STP") is used to derive the point code of the HLR based on tables associated with the MIN value. The VLR can send the registration request to the STP which then derives the appropriate HLR point code based on the MIN. The STP derives the address of the appropriate HLR based on the first six digits (corresponding to the NPA-NXX portion of the directory number).
During the period in which the mobile station is registered in the VLR, the mobile station may originate calls. During call origination, the MSC uses the stored MIN as the telephone number associated with the roamer. This value is used in populating certain SS7 is signaling parameters such as the "Calling Party Number" parameter.
While domestic automatic roaming is fairly common, international automatic roaming presents greater difficulties. Cellular subscribers from outside of the United States, Canada, and U.S. territories frequently do not enjoy the benefit of automatic roaming upon entering the country. Frequently, mobile subscribers cannot successfully register, which results in service being denied; or if they have successfully registered, toll calls may not successfully complete. Calls made to foreign countries (which are of particular interest to international roamers) are usually barred entirely. Accordingly, the utility of cellular service for an international roamer is drastically reduced. The problems associated with a mobile station registering are due to three problems: (1)The registration procedure necessary to establish a call often requires the existence of certain international business relationships; and (2) Various countries around the world have adopted different (and often incompatible) numbering plans; and (3) The frequent prohibition of international calls imposed upon subscribers by many U.S. cellular service providers.
The registration procedures require that the visited WSP have a direct or indirect business arrangement to provide service to the home service provider. While in the U.S. most cellular carriers have established roaming agreements directly with other carriers or utilize the service of clearinghouse to facilitate this, this is not often the case between international cellular service providers. Assuming that such roaming business arrangements are established, technical issues exist regarding the transfer of signaling information between the VLR and HLR. As previously described, this signaling information is carried by the SS7 network and requires that each entity be assigned an address known as a point code. Service providers outside the U.S., however, may not obtain point code addresses within the U.S. allocated numbering space. Rather, the signaling must be directed to an international gateway that can route such messages. Unfortunately, few, if any, international gateways have been established. This significantly limits the ability of a mobile station to automatically register.
Assuming that registration could successfully occur, the mobile station would then be authorized to initiate a call. Nevertheless, problems can occur with call establishment that hinders the utility of automatic roaming. One of these problems is traced to the differences between the telephone numbering plans of the U.S. and other countries which are not part of the NANP. When the mobile station originates a call, the MSC uses the stored MIN value as the calling party number to signal for call establishment to the PSTN. The local PSTN switch (or subsequent interexchange carrier switches) may examine the calling party number to ascertain the originating subscriber. Included in the examination is determining whether the NPA portion corresponds to a currently assigned area code as well as whether the NXX portion is properly formatted. If either of these portions corresponds to an invalid NPA-NXX-XXXX combination, the PSTN switch might deem the telephone number invalid. This might occur if the number is derived from the mobile station's MIN since it would be based on the numbering plan of the subscriber's country of origin. Many U.S. carriers would terminate processing of the call.
The final aspect that limits the utility of international roaming is the result of limitations imposed by many U.S. cellular service providers. Many such providers currently prohibit any international calls by any subscribers whether roaming or not, partly for concerns of losses due to fraud. It is anticipated that international roamers would frequently desire to phone their home country; prohibiting such calls obviously limits the utility of international roaming.